This invention relates generally to internal combustion engine ignition systems and, more particularly to a traveling spark ignitor having a long-life and associated firing circuity therefore.
Internal combustion engines have undergone many changes since their initial development at the end of the last century. Many of these evolutionary changes can be seen as a maturing of technology, with the fundamental principles remaining the same. Such is the case with the ignition system. Some of its developments include the replacement of mechanical distributors by electronic ones, increasing reliability and allowing for easy adjustment of the spark timing under different engine operating conditions. The electronics responsible for creating the high voltage required for the discharge have changed, with transistorized coil ignition (TCI) and capacitive discharge ignition (CDI) systems common today.
The need for an enhanced ignition source has long been recognized. Many inventions have been made which provide enlarged ignition kernels. The use of plasma jets and Lorentz force plasma accelerators have been the subject of much study and many patents. The primary weakness of some of the prior inventions has been the requirement for excessive ignition energy, which eliminates any possible efficiency enhancement in the engine in which they are employed. These higher ignition energy requirements have resulted in high rates of ignitor electrode erosion, which reduces the operating life of such ignitors to unacceptable levels.
The electrical energy required in these earlier systems, e.g., Fitzgerald et al., U.S. Pat. No. 4,122,816, is claimed to be more than 2 Joules per firing (col. 2, lines 55-63). This energy is about 40 times higher than that used in conventional spark plugs.
Matthews et al., infra, reports the use of 5.5 Joules of electrical energy per ignition, or more than 100 times the energy used in conventional ignition systems.
Consider a six cylinder 4-stroke engine operating at 3600 RPM, which requires firing three cylinders every engine revolution, or 180 firings per second. At 2 Joules per firing this is 360 Joules/second. This energy must be provided by the combustion engine at a typical efficiency of about 18% and converted to a suitable high voltage by power conversion devices with a typical efficiency of about 40%, for a net use of the engine fuel at an efficiency of about 7.2%. Fitzgerald requires a fuel energy consumption of 360/0.072 Joules/second, or about 5000 Joules/second just to run the ignition system.
To move a 1250 kg vehicle on a level road at about 80 km/hr (about 50 mph) requires an energy consumption rate of about 9000 Joules/second. At an engine fuel to motive force conversion efficiency of 18%, about 50,000 Joules/second of fuel will be consumed. Thus, the system employed by Fitzgerald et al, infra, will use about 10% of the fuel energy consumed to run the vehicle to operate the ignition system. This is greater than the efficiency gain to be expected by use of the Fitzgerald et al. ignition systems.
By comparison, conventional ignition systems use about 0.25 percent of the fuel energy to run the ignition system. However, the high energy employed in these systems causes high levels of erosion to occur in the electrodes of the spark plugs, thus reducing the useful operating life considerably. This shortened life is demonstrated in the work by Matthews et al., infra, where the need to reduce ignition energy is acknowledged although no solution is provided.
Additional attempts at solving this problem are shown in the work by Tsao and Durbin (Tsao, L. and Durbin, E. J., xe2x80x9cEvaluation of Cyclic Variation and Lean Operation in a Combustion Engine with a Multi-Electrode Spark Ignition Systemxe2x80x9d, Princeton Univ., MAE Report, (January, 1984)), where a larger than regular ignition kernel was generated by a multiple electrode spark plug, demonstrating a reduction in cyclic variability of combustion, a reduction in spark advance, and an increase in output power. The increase in kernel size was only six times that of an ordinary spark plug.
Bradley and Critchley (Bradley, D., Critchley, I. L., xe2x80x9cElectromagnetically Induced Motion of Spark Ignition Kernelsxe2x80x9d, Combust. Flame 22, pgs. 143-152 (1974)) were the first to consider the use of electromagnetic forces to induce a motion of the spark, with an ignition energy of 12 Joules. Fitzgerald (Fitzgerald, D. J., xe2x80x9cPulsed Plasma Ignitor for Internal Combustion Enginesxe2x80x9d, SAE paper 760764 (1976); and Fitzgerald, D. J., Breshears, R. R., xe2x80x9cPlasma Ignitor for Internal Combustion Enginexe2x80x9d, U.S. Pat. No. 4,122,816 (1978)) proposed to use pulsed plasma thrusters for the ignition of automotive engines with much less but still substantial ignition energy (approximately 1.6J). Although he was able to extend the lean limit, the overall performance of such plasma thrusters used for ignition systems was not significantly better than that of regular spark plugs and the sparks they produce. In this system, much more ignition energy was used without a significant increase in plasma kernel size. (Clements, R. M., Smy, P. R., Dale, J. D., xe2x80x9cAn Experimental Study of the Ejection Mechanism for Typical Plasma Jet Ignitorsxe2x80x9d, Combust. Flame 42, pages 287-295 (1981)). More recently Hall et al. (Hall, M. J., Tajima, H., Matthews, R. D., Koeroghlian, M. M., Weldon, W. F., Nichols, S. P., xe2x80x9cInitial Studies of a New Type of Ignitor: The Railplugxe2x80x9d, SAE paper 912319 (1991)), and Matthews et al. (Matthews, R. D., Hall, M. J., Faidley, R. W., Chiu, J. P., Zhao, X. W., Annezer, I., Koening, M. H., Harber, J. F., Darden, M. H., Weldon,2
W. F., Nichols, S. P., xe2x80x9cFurther Analysis of Railplugs as a New Type of Ignitorxe2x80x9d, SAE paper 922167 (1992)), have shown that a xe2x80x9crail plugxe2x80x9d operated at an energy of over 6J (2.4cm long) showed a very substantial improvement in combustion bomb experiments. They also observed improvements in the lean operation of an engine when they ran it with their spark plug at an ignition energy of 5.5J. They attributed the need of this excessive amount of energy to poor matching between the electrical circuit and the spark plug. This level of energy expended in the spark plug is about 25% of the energy consumed in propelling a 1250 kg vehicle at 80 km/hr on a level road. Any efficiency benefits in engine performance would be more than consumed by the increased energy in the ignition system. All of the above references are herein incorporated by reference.
Various aspects of the present invention overcome the above and other drawbacks in the art of ignitors and ignition systems for internal combustion engines. In one embodiment of the present invention, a plasma-generating device having a long-life is disclosed. As used herein, the term xe2x80x9cplasma-generating devicexe2x80x9d refers to an ignitor (spark plug) that generates a large volume of plasma. One specific type of plasma generating-device is a traveling spark ignitor (TSI). A TSI generates an initial spark between two electrodes due to a first high voltage between the electrodes. This initial spark creates a plasma that is then swept outwards due to both Lorentz and thermal expansion forces. As the spark is swept outwards, at least some of the gas present in the space between the electrodes is converted to plasma. Examples of TSI""s are disclosed in U.S. Pat. No. 5,704,321, filed Oct. 11, 1996, and U.S. patent applications Ser. No. 09/204,440, entitled High Efficiency Traveling Spark Ignition System and Ignitor Therefore, filed Dec. 2, 1998, both of which are incorporated herein by reference.
According to one illustrative embodiment, a plasma-generating device is disclosed. The plasma-generating device of this embodiment includes at least two spaced-apart electrodes including a first electrode and a second electrode having a discharge gap between them, and dielectric material filling a substantial portion of the space between the first electrode and the second electrode. The dielectric material has an upper surface and a lower surface and is spaced apart from at least a portion of the second electrode by a width of the lower surface, to define an air gap between the upper surface and the second electrode. The electrodes are dimensioned and configured and their spacing is being arranged such that when a sufficiently high first voltage is applied across the electrodes when the device is disposed in a combustion chamber of an engine, a discharge occurs and a plasma is formed between the electrodes at an initiation region. The plasma moves outwardly along the electrodes and away from the surface of the initiation region under both a thermal expansion force and a Lorentz force. The discharge is associated with, or results in part in, electrode ablation. The air gap may serve to help reduce this ablation by
In another illustrative embodiment, a traveling spark ignitor is disclosed. The traveling spark ignitor of this embodiment includes at least two spaced-apart electrodes including a first electrode and a second electrode having a discharge gap between them and dielectric material filling a substantial portion of the space between the first electrode and the second electrode. The dielectric material has an upper surface and a lower surface and is spaced apart from at least a portion of the second electrode by a width of the lower surface to define an air gap between the upper surface and the second electrode. The traveling spark ignitor may also include a co-axial connector to connect a co-axial cable to the ignitor. The electrodes are dimensioned and configured and their spacing is arranged such that when a sufficiently high first voltage is applied across the electrodes in a combustion chamber of an engine, a plasma is formed between the electrodes at an initiation region. The plasma moves outwardly along the electrodes and away from the initiation region due to a Lorentz force.
In another embodiment, a system of igniting a gaseous mixture of air and fuel in a combustion chamber of an internal combustion engine is disclosed. The system of this embodiment includes a traveling spark ignitor that includes at least two spaced-apart electrodes including a first electrode and a second electrode having a discharge gap between them and dielectric material filling a substantial portion of the space between the electrodes. The dielectric material is spaced apart from at least a portion of the second electrode to define an air gap. The system also includes means for alternatively producing a first and second potential difference between the electrodes, the first potential creating a plasma in an unfilled portion of the discharge gap at a plasma initiation region, the second potential sustaining a current through the plasma, whereby a magnetic field from the current interacts with an electric field from the potential difference between the electrodes to cause the plasma to move outwardly from the initiation region under a Lorentz force.